Exhaust gas recirculation (hereinafter EGR) is used in automobile diesel engines to control the emission of oxides of nitrogen (hereinafter NO.sub.x). In many instances, the amount of EGR applied to the engine is controlled or scheduled as a function of the induced air quantity entering the engine. While such an approach is fundamentally sound, it is difficult to implement because it requires either estimating or measuring the amount of airflow inducted into the engine. The maximum EGR is typically limited by exhaust smoke, or more precisely, exhaust particulate emissions. Because of this limitation, many systems increase the amount of EGR until the measured air/fuel ratio reaches a predetermined amount that decreases the oxides of nitrogen as much as possible while limiting the smoke emissions to some predetermined satisfactory maximum.
A recent U.S. Pat. No. 4,333,440 to Eheim describes an apparatus for controlling EGR in a diesel engine without resort to a measurement of the induced airflow or the air/fuel ratio. That system utilizes measurements of the speed and the fuel delivery quantities of a fuel injection pump for identifying and retrieving EGR control values stored in a memory as functions of the pump speed and fuel quantity values. The retrieved EGR values are utilized to control the EGR valve. Such a system, however, makes no provision for directly controlling the level of particulates emitted by the engine, which level may vary as a function of factors other than pump speed and fuel quantity. For instance, fuel quality, engine operating temperature, ambient air pressure and engine wear may cause variations in the level of particulate contained in the exhaust gas emissions for constant conditions of fuel pump speed and injection quantity.
Another U.S. Pat. No. 4,186,701 to Suzuki et al discloses an EGR control arrangement for maintaining an adequate balance between the suppression of NO.sub.x emission and the preservation of stable engine operation in a spark ignition engine. This balance is provided by monitoring the condition of combustion in the engine and developing therefrom a feedback signal for controlling the volume of recirculated exhaust gas. The feedback signal is provided by a probe which applies a voltage across a combustion chamber of the engine and uses the resulting ionic current as an indication of the combustion conditions. Specifically, the ionic current decreases as the amount of the retained combustion gas increases. The drawback to this approach as an EGR control resides in the fact that the probe measures combustion chamber conditions and not exhaust gas emissions. Reduction of NO.sub.x requires relatively high levels of recirculated exhaust gas yet reference to FIG. 2 of the Suzuki et al patent clearly reveals that the probe signal is of least sensitivity in that region.
Therefore, it is a principal object of the present invention to provide improved apparatus for controlling the recirculation of exhaust gas in a diesel engine. Specifically, it is desired to provide such control at least partly as a function of the level of particulates in the engine's exhaust gas stream.
It is a further object of the invention to provide an EGR system which provides improved control through use of a feedback sensor which measures exhaust gas emissions directly.
In accordance with the present invention, there is provided a method and apparatus for controlling the recirculation of exhaust gas in a diesel engine having an exhaust gas recirculation duct connected between an exhaust duct from the engine and the air intake duct to the engine. An EGR valve operatively associated with the recirculation duct is responsive to an EGR control signal for relatively opening and closing the recirculation duct to the passage of exhaust gas therethrough. A sensor is mounted in operative association with the exhaust gas stream passing through the exhaust duct for providing a signal indicative of the level of particulates in the exhaust gas stream. Control circuitry responsive to various signals indicative of engine operating conditions including the signal indicative of exhaust gas particulate level, provides an EGR control signal for regulating the EGR valve at least partly as a function of the sensed level of particulates in the exhaust gas stream. The engine operating condition signals to which the control circuitry responds typically are indicative of the speed of the engine or fuel injection pump, the engine load, and engine operating temperature. The particulate level signal is preferably provided by electrostatically sensing the passage of packets of charged particles by an electrically-passive, annular electrode positioned in or adjacent to the exhaust gas stream. Circuitry may be provided for occasionally calibrating the signal of the measured level of particulates in order to compensate for changes which may be occasioned by different fuels.
The control circuitry includes a memory for storing open loop EGR valve control signals as a function of speed and load. Additional memory means store particulate objective signals also as a function of speed and load. The sensed exhaust gas particulate level is compared with the appropriate particulate objective signal for the development of an error signal, which error signal is then used in an adaptive manner to adjust the open loop EGR control signal in a direction to diminish the particulate error signal. Such adaptive control of the EGR control signal as a function of the exhaust gas particulate level is further advantageous because a predeterminable relationship exists between exhaust particulate levels and NO.sub.x levels as a function of percent EGR. For depressed engine temperatures, as at start, provision is made for inhibiting EGR control.